The present invention relates to artifact rejection using signal-noise averaging and filtering techniques, and more particularly to averaging repetitive signals representative of repetitive noise-vulnerable physiologic events such as blood pressure pulses based upon substantially noise-immune heart-beat-related events such as ECG data or plethysmograph data from a pulse oximeter.
Ensemble-signal averaging (ESA) is known generally as a technique for removing severe noise from a repetitive input signal based upon the existence of a timing source that is substantially immune to noise. In the field of medical monitoring, such a technique is used in connection with measuring brain stem auditory evoked response, and with measuring ECG late potentials. Essentially, the idea is that noise will occur randomly with respect to the timing source, and will cancel out through such averaging if sufficient data is taken. In contrast, desired measured data, such as medical monitoring data, will not be random with respect to the timing source. Therefore, medical monitoring data will not cancel out through averaging, regardless of the amount of data points taken for averaging purposes.
Another type of medical monitoring involves the oscillometric method of noninvasive blood pressure (NIBP) measurement. Various methods of dealing with artifact have been proposed including those described in U.S. Pat. Nos. 4,949,710 to Dorsett et al. and 5,339,822 to Taylor et al. Generally speaking, conventional artifact rejection methods for measured physiologic pulsatile data focus on determinations for accepting or rejecting data corresponding to a given pulse. In other words, conventional methods focus on analyzing data, pulse-by-pulse, to learn whether to accept or reject.
However, until now, there has been no proposal for dealing with artifact in the form of relatively severe noise that is superimposed on NIBP oscillometric pulses in the oscillometric channel of an NIBP monitor. For purposes of the to-be-described invention, that relatively severe noise is not in synch with a to-be-described reference signal like an ECG signal. As will be understood, the to-be-described artifact rejector and artifact rejection method of the invention relies on the unwanted artifact/noise to be different in fundamental frequency and harmonics from the reference signal and its harmonics. In the context of oscillometric NIBP, it does not take much to result in severe noise because in ideal circumstances the largest oscillometric pulses are about 7-8-mmHg, and often such pulses are less than 1-mmHg. Environmental conditions that can produce severe noise in the context of oscillometric NIBP include actions that impact the blood pressure cuff such as patient motion, vehicular vibration (where the patient is being transported), and inadvertant physical contact between the patient and the health care professional(s) operating the monitor or treating the patient.
Also until now, ESA has never been tried on oscillometric NIBP because NIBP is not a method that can accept substantial extensions of cycle time due to patient discomfort associated with the occluding cuff. ESA has also not been tried on oscillometric NIBP because it could easily distort the shape of the clinically useful graphical information corresponding to a plot of pulse size (amplitude) vs. cuff pressure during an NIBP-measurement cycle (also called the NIBP cycle envelope).
Accordingly, it is a principal object of the present invention to provide an artifact-rejection mechanism which overcomes the drawbacks of prior-art proposals.
Another object is to provide such a mechanism which can be used to reject artifact causing severe noise in oscillometric NIBP.
Yet another object is to provide such a mechanism that allows for application of ESA to reject artifact in oscillometric NIBP.
A further object is to provide such a mechanism that is usable in oscillometric NIBP and preserves the shape of the NIBP cycle envelope.
Another object is to provide such a mechanism that will not substantially extend NIBP cycle time.
A still further object is to provide such a mechanism that will optimize NIBP cycle time.
Yet another object is to provide such a mechanism that will account for unusual time relationships that can occur between heart-beat related signal data such as ECG-signal data and oscillometric-pulse-signal data.
Another object is to provide such a mechanism that will determine when data that has been averaged has been adequately initialized.
In brief summary, one aspect of the invention is an artifact rejector for repetitive physiologic-event-signal data generated from physiologic-event-measuring equipment that includes a physiologic-event-signal averager in communication with such equipment. The artifact rejector of the invention is constructed to generate and store repetitive averaged physiologic-event-signal data based upon a substantially stable time relationship between corresponding physiologic-event-signal data and heart-beat-related-signal data, with the repetitive averaged physiologic-event-signal data including less noise than the repetitive physiologic-event-signal data. The artifact rejector generates and continuously updates an averaged-data template by storing such repetitive averaged physiologic-event-signal data for a preselected number of measured physiologic events. The averager preferably includes a sharp roll-off, low pass filter, and examples include a fourth-order Bessel filter, two cascaded, identical second-order Bessel filters, an elliptic filter, a Tchetschebyscheff filter, or finite impulse response filters. The heart-beat-related signal is preferably an ECG signal.
Another aspect of the present invention is a method of artifact rejection that includes averaging such repetitive physiologic-event-signal data based upon a substantially stable time relationship between corresponding physiologic-event-signal data and heart-beat-related-signal data, with the repetitive averaged physiologic-event-signal data including less noise than the repetitive physiologic-event-signal data.
These and other objects and advantages of the invention will be more clearly understood from a consideration of the accompanying drawings and the following description of the preferred embodiment.